Friction-stir welding is known as a method for joining different materials such as an aluminum alloy member and a steel plate (see e.g., patent literature 1 below). In friction-stir welding the aluminum alloy member and the steel plate together, the steel plate is first placed on a back-up jig and then the aluminum alloy member is brought to overlap the steel plate. Next, a joining tool is pressed against a joining part where the aluminum alloy member overlaps the steel plate, as the tool rotates, such that a joining pin (a projection) penetrates the joining part. In this state, the joining tool is moved along the joining part to subject the joining part (i.e., the aluminum alloy member and the steel plate) to friction-stir welding.
A steel plate is plated with a zinc to form a zinc film protecting the steel plate from corrosion, after which the zinc film is coated with a resin-based coating through electrodeposition to form the resin-based coating film. When the steel plate with the resin-based coating film is friction-stir welded to an aluminum alloy member at the joint therebetween, the resin-based coating film placed on the back-up jig can be stripped due to heat generated during the friction-stir welding. When the resin-based coating film is stripped from the steel plate, it is difficult to ensure corrosion resistance of the stripped region.
To address that problem, it is thought that after the steel plate is friction-stir welded to the aluminum alloy member, the steel plate and the aluminum alloy member are subjected to electrodeposition to form a resin-based coating film thereon. The aluminum alloy member may have threaded portions etc., for example. Thus, the electrodeposition to form the resin-based coating film on the steel plate and the aluminum alloy member involves the cost for masking process to cover the threaded portions etc. Therefore, there is a need for practical use of a technique for ensuring corrosion resistance of a region of the steel plate even if the region is stripped of the resin-based coating film when the steel plate is friction-stir-welded to the aluminum alloy member after the resin-based coating film is formed on the steel plate through electrodeposition.